This invention relates generally to the art of investment casting, and more specifically to ceramic shell molding techniques of precision casting wherein shell molds suitable for casting metal are prepared by building up layers of refractory material around disposable patterns which are subsequently removed from the mold.
As is known to those familiar with the art of investment casting, ceramic shell molds are prepared using patterns which are replicas of the parts to be cast in metal and which are formed of an expendable material. These patterns are attached to a sprue member to form what is known as a "set-up". The formation of the shell mold around the set-up is accomplished by dipping it into a refractory slurry of controlled viscosity followed by directional draining to coat the patterns uniformly. After draining excess slurry from the set-up, the slurry coating is sanded or stuccoed while wet with coarser refractory particles, such as by dipping the set-up into an air-fluidized bed of dry refractory material. The result is a coat of ceramic material having refractory particles embedded in the surface. This coat is hardened, usually by air drying at room conditions. After the first coat is sufficiently hard and dry, the steps of dipping, draining, stuccoing and drying are repeated until a refractory shell having a sufficient thickness to resist the stresses occurring in subsequent operations have been built up aroung the set-up. The usual shell thickness is from 1/8 inch to about 1/2 inch; although thicker or thinner shells may be formed for special situations. The set-up including the disposable patterns is then removed from the shell mold and the mold prepared for the casting operation.
The present invention is particularly concerned with an improvement in the ceramic shell molding technique and apparatus of U.S. Pat. Nos. 3,812,898 and 3,871,440 issued May 28, 1974 and Mar. 18, 1975, respectively, to Robert A. Horton. As disclosed in those patents, several pattern set-ups are processed as a group through the shell building operations. The several set-ups are arranged horizontally in ferris-wheel fashion between a pair of end plates. The entire array of circumferentially spaced pattern set-ups is rotated about a horizontal axis to cause each set-up to be moved about the axis between a lowered position in which the set-up is at least partially immersed in fluent ceramic shell material and a raised position.
The technique of applying refractory coatings to a plurality of set-ups in a single operation reduces the cost of making the shell molds and offers additional advantages which are of particular benefit to investment foundries whose operations are limited by the capacity of their melting furnaces and other equipment and which do not require the sophisticated mold-making apparatus of the size used by larger foundries.